The present invention relates to a bipolar transistor which can operate at a high operating frequency and with a high speed, more particularly, to a heterojunction bipolar transistor.
In a conventional transistor of this type, e.g., in an n-p-n heterojunction bipolar transistor, a semiconductor layer (collector layer) for forming a base-collector depletion layer consists of an n-type layer having a concentration lower than concentrations of a base layer, where the n-type collector layer has a uniform or an inclined concentration distribution. A heterojunction bipolar transistor (to be referred to as an "HBT" hereinafter) using a III-V Group semiconductor (e.g., GaAs) which has been widely developed in recent years has a similar impurity structure.
In this conventional structure, an electrical field intensity in the base-collector depletion layer which is mostly determined by an impurity concentration of the collector layer is significantly high. If, for example, the impurity concentration is 5.times.10.sup.16 /cm.sup.3, the electric field intensity of the base-collector depletion layer is increased more than 100 kV/cm when an appropriate bias voltage is applied to terminals of transistor for transistor operation. Therefore, under such a high electric field, an electron velocity in the depletion layer is determined as an "electron saturation velocity (Vs)" as is well known, and a corresponding collector transit time t.sub.C is given as follows: EQU t.sub.C =W.sub.C /2Vs
where W.sub.C is a base-collector depletion layer width.
In an Si bipolar transistor, since a ratio of a base transit time t.sub.B to a total delay time of an element is large and the collector transit time t.sub.C has minor contribution, almost no problem is posed. However, since the base transit time t.sub.B is very short in an AlGaAs/GaAs HBT or the like, contribution of the collector transit time t.sub.C poses a great concern.
Transport of electrons in a collector in a conventional AlGaAs/GaAs HBT will be described with reference to FIGS. 1(a) to 1(c). FIG. 1(a) shows an energy band diagram, FIG. 1(b) shows a layer arrangement, and FIG. 1(c) shows an electric field distribution in a collector depletion layer In FIGS. 1(a) and 1(b), reference numeral 1 denotes an emitter layer; 2, a base layer; 3, a collector layer; 4, a collector electrode layer; 5, a conduction band edge; 6 a valence band edge; 7, an energy band edge curve representing the bottom of an L valley; 8, an energy curve representing the bottom of a .GAMMA. valley in this case, equal to 5; and 9, electrons. In a normal state wherein a base/collector junction forms a p.sup.+ -n diode, an electric field intensity is maximized just inside of the collector layer on a junction plane as indicated by an electric field intensity profile line 10 in FIG. 1(c). Therefore, since an energy of the electrons 9 injected from a base into the collector is increased higher before they run several hundreds .ANG., the electrons 9 enter into and are distributed not only in a .GAMMA. valley but also in the L and X valleys having high energies. This is inherent because, as long as a base/collector bias voltage is plotted on the reverse biasing side with respect to about +0.5 V, i.e., it falls within an active region of the transistor, electron energy exceeds easily an energy between the L valley and a lower .GAMMA. valley is about 0.3 eV in GaAs.
When the electrons 9 move to the L or X valley, an electron velocity becomes a so-called saturation velocity Vs. When GaAs is used, the saturation velocity Vs is around 7.times.10.sup.6 cm/sec. Since an effective mass of the electrons entering into the L or X valley becomes larger than that of the electrons distributed in the .GAMMA. valley, the electron velocity in these valleys becomes smaller than that obtained in the .GAMMA. valley.
Recently, "A Proposed Structure for Collector Transit-Time Reduction in AlGaAs/GaAs Bipolar Transistors" by C. M. Maziar et al. (IEEE, Electron Dev. Lett. EDL-7, No. 8, pp. 483-485, 1986) describes a proposal of reducing an electric field intensity in the collector depletion layer on the base side and increasing an electron velocity. That is, by changing a conductivity type of a collector layer from a conventional n-type to a p-type, the peak of an electric field is shifted to the side of a collector electrode layer.
However, a degree of such an improvement is only several tens% in terms of a collector transit time. This is because, although an overshoot effect (i.e., a phenomenon in which an electron velocity is transitionally increased) is partially utilized, this effect is not significant as a whole since the electrons 9 transits a region of 50% or more of a base-collector depletion layer at a saturation velocity.